"Now just sitting in a dining hall and unwrapping your sandwiches is considered to be an optional extra, it's disgraceful, it's shocking," said Patrick Roach, deputy general secretary of NASUWT. "Parents should be appalled in just the same way that we're appalled."

While the NASUWT survey (which is not yet published) found 14 parents reported being charged between 10 pence and 60 pence per day for their children to eat home-made lunches at school, union bosses believe the practice is far more widespread than this and warn that it may become more common as schools continue to struggle to cope with shrinking budgets.

Chris Keates, general secretary of the union, said she had heard of cases in which schools were charging more, such as a secondary school in the south east charging £1.80 per day and a primary school in Yorkshire charging £1.

"Schools are justifying it by saying, 'you're having to be supervised to eat your lunch and therefore if you're not having a school meal, you've got to contribute towards that supervision because you'll be in the dining room, sitting there,'" she said.

A spokesman for the Department of Education told TES it was "absolutely unacceptable" for schools to charge children to bring in packed lunches.

"If schools are looking to exploit loopholes to get money out of parents we will investigate and make sure those loopholes are closed," he said.

The survey also reveals that nearly three-quarters of teachers have seen pupils come to school hungry. More than a quarter of teachers said they have had to step in and provide food for children and more than half said they had seen their schools do the same.

For families without financial worries paying for their kids' pleasure of a packed lunch may be annoying. But for families who struggle to make ends meet it's much more than that — and it's the children who'll suffer the most.

Keates expressed her concern that "teachers and schools are being left to pick up the pieces of callous fiscal and social policies."

"As the survey shows, poverty and homelessness take an enormous physical and emotional toll on children. They often cannot concentrate when they are in school because they are tired, hungry and anxious," she said. "Children living in poverty are more likely to suffer from low confidence and behavioural issues. Homelessness leads to ill health and absenteeism when the distance and cost of travelling to school from temporary accommodation is prohibitive. Teachers and support staff are mending clothes and washing uniforms, providing food and equipment.

"It is hardly credible that this is happening in one of the world's largest economies."